1. Technical Field
The present invention generally relates to video digital signal processing (DSP) systems for generating vestigial-sideband (VSB) signals and, more particularly, to a system and method for adaptively balancing quadrature modulators in a video digital signal processing system for generating vestigial-sideband signals.
2. Description of Related Art
Frequency translation is the basic idea behind radio communications. That is, frequency translation allows the generation of signals in radio communications systems with desirable transmission characteristics, such as antenna size, freedom of interference from similar information sources, line-of-sight to long-range propagation, and freedom of interference from noise sources. Moreover, frequency translation permits the efficient utilization of open and closed propagation media by many simultaneous users and/or signals.
One of the most used forms of frequency translation is linear modulation, the most common of which is amplitude modulation. In general, amplitude modulation consists of varying the magnitude of the carrier signal in direct correspondence to the instantaneous fluctuations of a modulating signal source, as described in Electronics Engineers"" Handbook, 3rd Edition, Donald G. Fink and Donald Christiansen, Eds., 1989, Section 14, Modulators, Demodulators, and Converters, hereby incorporated by reference.
Variations of the basic amplitude modulation process have been developed to achieve more efficient spectrum utilization and to reduce transmitter power requirements. An example is a vestigial-sideband modulation system. In a vestigial-sideband modulation system, information transmitted by an amplitude modulated carrier is wholly contained in the modulation sidebands. The transmission of the carrier energy adds no information and, moreover, each sideband contains the same information, and only one is required to transmit the xe2x80x9cintelligencexe2x80x9d. As such, elimination of one sideband can effect a substantial transmitter power saving. Elimination of one sideband is typically known as vestigial-sideband transmission modulation. The unwanted sideband is generally filtered out at a transmitter or receiver and is known as transmitter attenuation or receiver attenuation, respectively. The primary objective of vestigial-sideband transmission modulation systems is to conserve spectrum in the transmission medium.
Presently, vestigial-sideband signals, for example, television VSB signals, are transmitted using analog modulation techniques. For example, a typical television VSB signal transmission system uses traditional surface acoustic wave (SAW) filters for suppressing the unwanted sideband. SAW filters have errant imperfections, however, because they are temperature and power sensitive.
The present systems do not use digital signal processing techniques, which are less temperature and power sensitive than analog processing techniques, to generate television VSB signals because DSP processors have only recently achieved the speed, complexity, and price requirements to perform video digital signal processing. With the advent of digital signal processing, however, there are new demands on quadrature modulators performance (balance).
Using DSP techniques to generate video VSB signals has created a new need for balancing quadrature modulators. That is, imbalances in the quadrature modulators typically result in the lower sideband being xe2x88x9240 dB below the desired signal. It is desirable, however, that spurious products, such as the lower sidebands, be xe2x88x9260 dB below the desired signal in a video VSB system. Quadrature modulator errors include gain imbalance, phase imbalance, and DC offset errors.
Referring to FIG. 1, there is shown a conventional quadrature modulator 100. Quadrature modulator 100 comprises two bi-phase modulators 102 and 104, a phase shifter 106, and a combiner 108. The quadrature modulator 100 generally operates as follows. The modulator 102 modulates a first carrier signal 110 using an in-phase (I) baseband signal 112 to produce a modulated in-phase signal 114. The phase shifter 106 receives the first carrier signal 110 and generates a second carrier signal 116, wherein the second carrier signal 116 is in quadrature with the first carrier signal 110 (that is, the second carrier signal 116 is 90xc2x0 out of phase with the first carrier signal 110).
The modulator 104 modulates the second carrier signal 116 with a quadrature (Q) baseband signal 118 to produce a modulated quadrature signal 120, wherein the modulated quadrature signal 120 is in quadrature with the modulated in-phase signal 114. The modulated in-phase signal 114 and the modulated quadrature signal 120 are then combined in-phase by the combiner 108 to produce a radio frequency (RF) output signal 122.
The in-phase baseband signal 112 and the quadrature baseband signal 118 typically range in frequency from approximately 0 Hz to 2.5 MHz. The frequency of the first carrier signal 110 is approximately between 100 MHz and 1000 MHz. These frequencies are provided for illustrative purposes only, and may be other values. As will be appreciated, the transmit frequency of the RF output signal 122 is substantially equal to the frequency of the first carrier signal 110.
Modulators 102 and 104 include imperfections that may result in the generation of undesired mixing products. That is, because of the imperfections in the modulators 102 and 104, the RF output signal 122 may include undesired signal components having frequencies equal to the frequency of the in-phase baseband signal 112, the frequency of the quadrature baseband signal 118, the frequencies of the first and/or second carrier signals 110 and 116, and/or combinations of these frequencies.
The signal components having frequencies equal to the frequencies of the first and second carrier signals 110 and 116 are difficult to eliminate from the RF output signal 122 because their frequencies are so close to the transmit frequency of the RF output signal 122. These undesired signal components are commonly known as carrier leakage signals, or simply carrier leakage, because they originate from the first and second carrier signals 110 and 116, respectively.
Referring to FIG. 2, there is shown a graph 200 in the frequency domain of the energy output of the circuit of FIG. 1. The RF output signal 122 has a carrier leakage having a frequency of, for example, wc 202, an undesired sideband having a frequency of+for example, wcxe2x88x92wm 204, and a desired sideband having a frequency of, for example, wc+wm 206. The carrier leakage frequency wc 202 and undesired sideband wcxe2x88x92wm 204 represent wasted power and interference, and produce other undesirable effects such as jitter. As such, it is desirable to suppress the carrier leakage frequency wc 202 and the undesired sideband frequency wcxe2x88x92wm 204.
It is possible to eliminate the carrier leakage and undesired sideband by applying appropriate DC offsets, amplitude and phase balancing to the quadrature modulator. Also, carrier leakage and undesired sidebands can be eliminated by appropriately selecting component values to thereby balance the bi-phase modulators contained in quadrature modulators.
The application of DC offsets to the quadrature modulator and the selection of component values to balance the bi-phase modulators, however, do not represent complete solutions since carrier leakage varies with many factors, such as temperature, frequency, load impedance, and carrier power.
Some radio communications systems attempt to resolve the above problem of suppressing carrier leakage and undesired sidebands. One such system is disclosed in U.S. Pat. No. 5,162,763 to Morris. Morris is directed to a single sideband modulator for translating low frequency baseband signals directly to radio frequency in a single stage. Morris discloses monitoring amplitude of the RF output of the single sideband modulator, comparing it with the baseband signals, and generating continuous control signals to keep a local oscillator breakthrough and image sidebands down to a low level. Morris further discloses adjusting the DC offsets at the baseband inputs to the balanced modulators to cancel carrier breakthrough. Morris is not, however, directed to a video digital signal processing system for generating video vestigial-sideband signals and, as such, Morris does not address the problems of balancing quadrature modulators in a video DSP system, for example, balancing gain imbalance and phase imbalance. Moreover, Morris requires the generation of control signals and monitoring of the amplitude of the RF output to control imperfections in the quadrature modulator.
Other radio communications systems attempt to resolve the above problem of suppressing carrier leakage and undesired sidebands by comparing signal vectors. One such system is disclosed in U.S. Pat. No. 5,396,196 to Blodgett. Blodgett is directed to a carrier leakage suppression circuit which suppresses carrier leakage in a signal processing device which modulates a carrier signal with an in-phase baseband signal and a quadrature baseband signal to generate an RF output signal. The carrier leakage suppression circuit operates by imparting a first signature (a first code) to the in-phase baseband signal and a second signature (a second code) to the quadrature baseband signal prior to modulation of the carrier signal. In-phase and quadrature carrier leakage components in the RF output signal are isolated and measured by respectively correlating the RF output signal with the first and second signatures. An in-phase offset and a quadrature offset are generated as a function of the measurement of the in-phase and quadrature carrier leakage components. The in-phase baseband signal is combined with the in phase offset and the quadrature baseband signal is combined with the quadrature offset to thereby suppress carrier leakage. Blodgett is also not directed to a video digital signal processing system for generating video vestigial-sideband signals and, as such, Blodgett does not address the problems of balancing quadrature modulators in a video DSP system.
Accordingly, there is a significant need for a system and method for balancing quadrature modulators in a video digital signal processing system for generating vestigial-sideband signals wherein carrier leakage is suppressed even when the carrier leakage changes due to temperature, frequency, load impedance, and carrier power.
The limitations cited above and others are substantially overcome through the system and method disclosed herein.
The present invention overcomes a problem that arises when applying digital signal processing techniques to generate (or synthesize) a vestigial television signal. The DSP synthesized modulator uses an analog quadrature modulator to modulate the complex baseband signal to a standard television IF frequency, for example, 45.7 MHz, for the NTSC standard. IF frequencies as high as 1 GHz may be used to simplify the overall design of the modulator. The modulator maintains spurious products below xe2x88x9260 dBc. The quadrature modulator is balanced by fine tuning (adjusting) a quadrature modulator compensator. The invention uses feedback techniques to constantly xe2x80x9cbalancexe2x80x9d the quadrature modulator under operation conditions and over temperature range fluctuations. Balance is achieved by sampling the undesired signals (distortion) on the output, and tuning the quadrature modulator compensator until the distortion is minimized.
A digital signal processing vestigial-sideband television modulator in accordance with the teachings of the present invention comprises means for receiving a television signal; means for digitizing the television signal to generate a video signal; means for digitally processing the video signal to generate a complex baseband signal; means for converting the complex baseband signal into an in-phase baseband signal and a quadrature baseband signal; means for modulating the in-phase baseband signal and the quadrature baseband signal to respectively generate a modulated in-phase baseband signal and a modulated quadrature baseband signal; means for combining the modulated in-phase baseband signal and the modulated quadrature baseband signal to produce a vestigial radio frequency television signal; means for tapping off intermodulation distortion of the vestigial radio frequency television signal; means for filtering the intermodulation distortion of the vestigial radio frequency television signal; means for detecting the power level of the intermodulation distortion of the vestigial radio frequency television signal; and means for compensating imbalances in the modulating means using the detected power level of the intermodulation distortion of the vestigial radio frequency television signal.
A method in accordance with the teachings of the present invention for compensating imbalances in a digital signal processing vestigial-sideband television modulator is characterized by the steps of receiving a television signal; digitizing the television signal to generate a video signal; digitally processing the video signal to generate a complex baseband signal; converting the complex baseband signal into an in-phase baseband signal and a quadrature baseband signal; modulating the in-phase baseband signal and the quadrature baseband signal to respectively generate a modulated in-phase baseband signal and a modulated quadrature baseband signal; combining the modulated in-phase baseband signal and the modulated quadrature baseband signal to produce a vestigial radio frequency television signal; tapping off an undesired sideband of the vestigial radio frequency television signal; filtering the undesired sideband of the vestigial radio frequency television signal; detecting the power level of the undesired sideband of the vestigial radio frequency television signal; and compensating imbalances in the quadrature modulator using the detected power level of the undesired sideband of the vestigial radio frequency television signal.
An apparatus for balancing quadrature modulators in a digital signal processing system in accordance with the teachings of the present invention comprises a receiver for receiving intermodulation distortion of a vestigial radio frequency television signal from the digital signal processing system; a filter for filtering the intermodulation distortion of the vestigial radio frequency television signal; a detector for detecting the power level of the intermodulation distortion of the vestigial radio frequency television signal; and means for compensating imbalances in the quadrature modulators using said detected power level of the intermodulation distortion of the vestigial radio frequency television signal.
The above features and advantages of the present invention will be better understood from the following detailed description taken into conjunction with the accompanying drawings.